1. Field of the Invention
The present invention generally relates to heat storage dishes, and particularly to a system of pressure relief for heat storage dishes.
2. Description of Background Information
In environments where food is prepared and cooked in a central location and distributed and served to consumers who are remotely located, such as in hotels, in aircraft and in institutional settings such as hospitals and nursing homes, there is often a delay between the time that the food is prepared, cooked and subsequently placed on a plate or other serving dish, and the time that the food is eventually presented to the consumer for consumption at a remote location, such as a hotel room, hospital room, on aircraft, etc. Accordingly, by the time the food is presented to the consumer, the food can become cold unless special measures are taken to keep the food hot. Various approaches to such meal service problems encountered in such service environments, sometimes referred to as "satelliting" have been employed in the food service and container industries.
One approach to solving such problems associated with the service of meals involves the use of heat retentive servers, which are serving trays constructed and adapted to keep food hot for a longer period than such food maintained in an ordinary room-temperature environment, serving trays having insulated portions therein, and/or serving containers which retain heat. Heat retentive servers commonly comprise serving trays having insulated portions therein, and/or serving containers which retain heat. Such containers typically are adapted to receive a plate containing portions of a meal which are to be kept hot. Such servers typically include an insulated base portion and an insulated dome portion, which together cooperate to define an insulated enclosure which is adapted to receive a plate having such heated meal portions thereon, and maintain the plate and the meal portions in an insulated environment. In some instances, the heat retentive server can include a portion which acts as a heat storage "battery", or a heat sink.
Such heat retentive servers can be designed to support dishware, which in turn holds a portion of a meal which is to be kept hot. In such circumstances, such a base is commonly called a "pellet" base, and the entire system, i.e., the base, dome and plate, is referred to as a "pellet system". When a heat sink is incorporated into a server base and the base supports a food-carrying dish, such as a plate, the base can be referred to as a plate server.
The heat sinks in such systems can include, e.g., a phase-change core, such as that disclosed in U.S. Pats. Nos. 4,982,722 and 4,246,884. In other approaches, a solid heat sink can be used.
In the past, heat retentive servers have employed convection or conduction heating in order to either heat a food service dish or heat a storage battery during food service operations. U.S. Pat. No. 5,603,858, issued Feb. 18, 1997 to WYATT, and assigned to Aladdin Synergetics, Inc., of Nashville, Tenn., the entirety of which is hereby incorporated by reference as though set forth in full herein, discloses a heat retentive server that is capable of being heated by induction.
Recently, induction heating of heat retentive servers as well as inductively heated servers and serving systems have been introduced by Aladdin Synergetics, Inc., Nashville Tenn. and are experiencing rapid and widespread acceptance in the marketplace, even revolutionizing the industry. Whereas conventional systems required long lead times, inductively heated servers and server heating systems provide apparatus and methods whereby individual pellets can be heated very rapidly, for example, in periods of 5-10 seconds and can be employed to keep food hot for longer than 1 hour. Speed, therefore, is an important advantage in using such systems, in which individual pellets are rapidly heated at the start of a tray line. Another advantage of such systems is their ability to direct heat up toward the food while reducing waste of heat energy through the bottom of the server. Such systems are disclosed in U.S. Pat. No. 5,603,858 to WYATT, incorporated above by reference.
It is believed that Aladdin's systems have prompted others to attempt to enter the marketplace with inductively heated servers. Thus, another inductively heated server is disclosed in U.S. Pat. No. 5,611,328 to McDERMOTT, filed Sep. 19, 1995 and issued Mar. 18, 1997, in which a metal disk substantially fills the entire interior volume of the server and in which it is believed that substantial heat may be lost through the bottom of the server. The server is produced by insert molding and the structure surrounding the metal disk is seamless.
For reasons of safety and ease of handling, the heat storage battery may be enclosed within an airtight chamber. When the battery is heated, however, the surrounding air is also heated and expands. Thus, there exists the danger that if overheated, the pellet might burst. Different methods have been developed to deal with the increases in pressure within the pellet chamber of conductively or conventionally heated servers.
SCHNEIDER, U.S. Pat. No. 4,059,096, discloses an imperforate pellet enclosed in reinforced chamber. The pellet has reduced thickness in the center region which overlies a raised portion in the lower surface of the enclosure. It is the stated intent of the SCHNEIDER invention to reinforce the pellet enclosure in order to reduce the chance of bursting.
LANIGAN et al., U.S. Pat. No. 3,837,330, discloses a heat retentive server in which essentially the entire lower surface of the pellet enclosure is concave. Further, through an opening in the center of the pellet, the upper and lower surfaces of the enclosure are welded together. In this fashion, the stress caused by excess internal pressure causes reversible deformation of the heat retentive server.
ROTHSCHILD, U.S. Pat. No. 4,086,907, also discloses a heat retentive server with a concave bottom surface. Additionally, however, the lower surface of the enclosure is provided with rectangular indentations or corrugations to permit controlled deformation and allow the pellets to stack whether hot or cold.
MURDOUGH et al., U.S. Pat. No. 3,734,077, discloses a heat retentive server in which the pellet is provided with an opening in the central portion. The upper surface of the enclosure extends downward into the opening, and the lower surface of the enclosure extends upward into the opening, such that, in the central portion, the upper and lower surfaces are either in contact, or at least in close proximity. Upon buildup of excess pressure in the chamber, the upper and lower surfaces move slightly apart from one another, thus relieving the excess pressure.
KREIS, U.S. Pat. No. 3,557,774, also discloses a heat retentive server in which the pellet is provided with an opening in the central portion. Here, however, the upper surface of the enclosure is flat, and the lower surface extends upward through the opening. The upper and lower walls are welded together to strengthen the server against bursting.
Each of these methods is suited for conductively or connectively heated servers, which ordinarily take 60 to 90 minutes or more to heat to operational temperature. It is doubtful that any of these methods would work effectively for inductively heated servers, in which the temperature of the pellet can reach operational temperature very rapidly, in seconds. Further, if the heat retentive server, through excessive use, or through misuse, should develop small cracks, small amounts of water might seep in, which upon evaporation by heating, can cause a significant pressure increase. Due to the rapid temperature rise, there is available very little time in which the excess pressure can escape through the cracks.
The use of valves to limit pressure buildup in closed containers is known. For example, KOLOSOWSKI, U.S. Pat. No. 5,228,384, discloses a double-boiler which is a vented double-walled container wherein the vent optionally comprises a spring-loaded valve as a means of pressure relief The vent disclosed has a threaded valve housing onto which a threaded valve cap is placed. To permit threading the valve cap on the valve housing, the housing extends from the boiler cap on which it is mounted. By means of a spring which bears upward on the valve cap, a plug is brought to bear downward on a venting hole, thus sealing the hole for normal operation of the boiler. The valve itself is on a threaded cap used for covering the filling hole of the double boiler.
CLARKE et al., U.S. Pat. No. 5,137,050, discloses a spring-biased pressure relief valve for use with cryogenic pumps. The seal is made resistant to leaks caused by dust by the use of an annular projection on either the housing or the valve closure, and an o-ring on which the annular projection is brought to bear.
VANDENBERK, U.S. Pat. No. 4,990,247, discloses a spring-biased pressure relief valve for use in liquid filters such as engine oil filters. The part analogous to the valve stem consists of two pairs of oppositely situated legs with hooks for engaging the cylindrical spring. The valve also comprises a bellows and a flow restrictor.
BARNARD, U.S. Pat. No. 4,574,836, discloses a pressure relief valve with a bypass indicator and re-settable means to prevent closing of the valve after it has opened.
KELLY, U.S. Pat. No. 2,784,737, discloses a pressure relief valve for use on axle roller bearings of railroad rolling stock. The outlet portion of the valve housing is rabbeted to accommodate a metal ring. The closure end of the valve stem protrudes beyond the valve seat. The head of the valve stem has a stop member to prevent the valve from becoming stuck in the closed position should the valve closure be struck from the outside.
PURDOM, U.S. Pat. No. 5,577,740, discloses a container which includes a thermally activated diaphragm which may be fastened to the container with an adhesive material. The diaphragm comprises two layers, a thermally fusible layer, and a support layer. In the presence of excess temperature, the thermally fusible layer fuses and releases the support layer.
RAGUSA et al., U.S. Pat. No. 4,859,822, discloses a mnicrowaveable container, having a lid which optionally includes a vent, which vent may be a suitable one-way valve, for releasing steam or vapor generated during microwave heating of the food in the container.
FLOYD et al., U.S. Pat. No. 4,672,996 discloses a self regulating valve which can be used as a pressure relief valve for explosion prevention in microwave system-based, closed vessel digestion procedure. The valve includes a pressure-deformable, resilient wall member having a fluid vent port, and an obstructing member that cooperates with the wall member to open and close the valve.
WILSON et al., U.S. Pat. No. 4,664,287, provides for a pressure relief vent comprising a flat rupturable diaphragm. The diaphragm is attached to the outside surface of the container along a perimeter and at a spot location within the perimeter. In one embodiment, fluid is permitted to flow between the diaphragm and the outer surface of the container. In another embodiment, the heart-shaped diaphragm is welded to the container. In another embodiment, the spot location produces a stress concentration, allowing the diaphragm to rupture at a predetermined pressure.
DOYLE et al., U.S. Pat. No. 3,659,584, discloses a disposable metallic double-boiler An outer chamber contains water or other cooking fluid, and an inner chamber contains food. In order to use the device, an adhesive tape is removed from the lid, the container is pierced, and the tape is replaced. The container is then heated on a stove. The replaced adhesive tape acts to retain the cooking vapors inside the container, but will be forced off the opening if the internal pressure becomes too high.